These days, more than 50% of patients suffering from cancer receive radiotherapy treatment. This treatment may be sufficient in some cases. It may be, however, that the medical team responsible for tracking the patient feels that a surgical ablation of the tumor must follow the radiotherapy treatment.
For nearly a century, radiotherapy has formed a part of the therapeutic arsenal for cancer illnesses. After a few early stutters, this technique is now well controlled and forms, with chemotherapy and surgery, the most widespread cancer treatment, culminating in a large number of recoveries.
The principle of radiotherapy is simple: it entails exposing the cancerous cells to an ionization, that is to say an emission of radiation, which will alter the composition of the genetic information of the cancerous cells. These days, the specialists have at their disposal a very wide range, in terms of quality and quantity, of ionizing radiations.
This genetic material contained in DNA form undergoes transformations which render the irradiated cell incapable of reproducing. This kind of sterilization thus reduces the anarchic reproduction of these malignant cells which are responsible for the cancer. Of course, the normal cells can also be affected by these radiations, but their rates of repair are greater than those of the cancerous cells, which thus produces a so-called differential effect. This differential effect explains the benefit of radiotherapy for the sick. Thus, the mission of the radiotherapist is to succeed in killing the cancerous cells and preserve the healthy cells of the patient.
Radiotherapy can thus be prescribed as a curative treatment, namely to destroy the tumor, as palliative treatment to mitigate the pain, or even as auxiliary treatment to prepare or complement a surgical intervention or chemotherapy.
Before the treatment proper can be followed, the radiotherapy has to be prepared. For this, the location and the dose of radiation which will be administered to the patient have to be determined. In a first step, the treatments are prepared using a scanner, which makes it possible to set the limits of the area that has to be irradiated. A three-dimensional reconstruction of software images will make it possible to locate the tumor very precisely. The patient will have to remain immobile, in order for the locating of the area to be as accurate as possible.
Once the area to be treated is determined, the practician applies to the patient small dots, tattooed or painted, notably using “fuchsine”, to identify the area to be treated. This first step takes 30 minutes to an hour. It is at this stage that the positioning lasers are used.
The second step consists in analyzing all the data acquired by the machine for the radiotherapist and the radiophysician to determine the treatment apparatus to be used, the distribution of the dose, the size, the number and orientation of the irradiation fields, which are most appropriate. The quantity of radiation prescribed depends on the age and the state of health of the patient, as well as the location and the type of cancer.
The total dose is then divided up into smaller doses, because the effect is cumulative. The divided-up doses will then be administered in various sessions, to each of them. These sessions will be spread on average over two to seven weeks.
The positioning lasers equip the diagnostic rooms—step 1—on the one hand, and the radiotherapy treatment rooms—step 2—on the other hand. They can be of two different types.
In the diagnostic rooms, they are used to guide the manipulators which enable the latter to apply the marking to the skin of the patients: this is the first step of the treatment.
In the treatment rooms, they are used to position the patient in the accelerator: this is the last step of the treatment.
The diagnostic rooms also house the virtual simulation. It is in these rooms that the tumor will be perfectly identified and located using an appropriate machine, the scanner.
A standard installation of known type consists of 5 moving lasers placed on either side of the table of the scanner which will generate 5 laser beams, in this case one beam per laser. The five lasers are driven by a computer system present in the control room.
The lasers form three distinct planes, namely a horizontal plane, a substantially transversal vertical plane, these two planes forming a cross, and yet another vertical plane which is generally perpendicular in the axis of the table, the so-called sagittal axis.
The abovementioned vertical beams have to be merged in a single plane. This plane must be perfectly vertical and parallel to the vertical plane of the machine isocenter. The horizontal beams have to be merged in a single plane. This plane must be perfectly horizontal and perpendicular to the vertical plane of the machine isocenter. The sagittal beam has to be perfectly vertical and perpendicular to the vertical plane of the machine isocenter,